An aqueous ink contains various kinds of water soluble dyes. These aqueous inks have excellent solution stability, but have bad water resistance when used in a written trace or in a printed image. Especially, it is more difficult to provide the excellent water resistance to the aqueous ink right after writing down.
Various improvements have been proposed to overcome the above undesirable water resistance, for example, Japanese Pat. Publn. (unexamined) Nos. 31526/1976, 115306/1978, 116762/1981, 131678/1981, 10658/1982, 30772/1982, 102972/1982, 198768/1982, 202359/1982 and 1764/1983 and Japanese Pat. Publn. (examined) No. 6581/1965. Among them, the use of a direct ink was proposed, but this ink does not have sufficient water resistance and does not have good solubility at a low pH solution. A solution of a high pH hydrolyzes an acetate fiber used in a storage material of a felt tip pen container to lower physical properties of the ink or dye. Also the dye does not have a good color tone other than a black one. However the black dye may increase the viscosity or gelation of the ink.
An ink containing a water insoluble dye dissolved in an organic solvent provides sufficient water resistance. It, however, spreads easily and also has an irritating smell because of using an organic solvent.
Also, an emulsion or dispersion ink using the water insoluble dye or a water insoluble pigment was proposed. It, however, has an insufficient stability with time and may have a plugging of a capillary passageway of the storage material.
There exists an ink by which a written trace is made water insoluble with time because of a reaction of the written trace with light or oxygen in the air. It, however, takes a long time to appear the water resistance, and this ink has a possibility to change a color.
Furthermore, it was also proposed that an ink was prepared by adding an additive (including a polymer) increasing the water resistance to the water soluble dye. This ink is not expected to have the excellent resistance right after writing down. The ink is not stable to a pH change so that it has to be kept in a small range of pH. The additive ink having higher water resistant has a bad storage stability.